
Then \(\det A=1\)  is the Wronskian determinant of the fundamental system \(\phi _1, \phi _2\)  and the Floquet multipliers \(\rho _{1,2} = \frac{1}{2} \left((A)\pm \sqrt{(A)^2-4}\right)\)  are the eigenvalues of \(A\)  with corresponding eigenvectors \(v_{1}=(v_{1,1}, v_{1,2})\in 2\)  and \(v_{2}=(v_{2,1}, v_{2,2})\in 2\) . Thus, \(\Psi _{j}(x)=v_{j,1}\phi _{1}(x)+v_{j,2}\phi _{2}(x)\) . By Hill's theorem (see [1]}) we know that
\(0\in \sigma (L) \qquad \Leftrightarrow \qquad {(A)}\le 2.\) 
